Optical device for scanning an optical record carrier

ABSTRACT

An optical device for scanning an optical record carrier ( 20 ) has a radiation source ( 25 ) that can emit two radiation beams of different wavelength along different optical paths ( 28, 29 ). The emitted radiation is focused on the record carrier and reflected back to a detection system ( 40 ). A beam combiner  43  is arranged in the optical path between the radiation source and the detection system for combining the two radiation beams on one optical path, thereby allowing the use of a single detection system for both radiation beams. The beam combiner includes a grating that passes one of the beams undiffracted and diffracts the other beam mainly in the first order. The profile of the grating lines show alternating slanting grooves and slanting lands.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an optical device for scanning an opticalrecord carrier, and to an optical grating for use in such a device. Moreparticularly, the device includes a radiation source for generating afirst radiation beam of a first wavelength traveling along a first pathand a second radiation beam of a different, second wavelength travelingalong a different, second path, a photo-detection system, an opticalsystem for guiding the radiation beams via the optical record carrier tothe photo-detection system, and a grating for combining the first andsecond beams such that they substantially coincide on thephoto-detection system. Scanning may refer to reading, writing anderasing information from the record carrier.

2. Description of the Related Art

The increasing demand for storage capacity has led to the development ofnew optical scanning devices and matching record carriers having anincreased storage capacity. As a consequence, old, low capacity recordcarriers and new, high capacity record carriers are simultaneouslyavailable on the market. For compatibility reasons, a scanning devicefor a new record carrier should be able to scan both old and new recordcarriers. This requires adaptation of the device to handle the differentformats of the record carriers. As an example, a scanning devicedesigned for scanning both the newer record carrier of the DVD type andthe older writable record carrier of the CD-R type must form a 650 nmwavelength radiation beam for scanning the DVD and a 780 nm wavelengthradiation beam for scanning the CD. The optical system of such a playerincludes two diode lasers, one for 650 nm radiation and one for 780 nmradiation. To reduce the cost of the optical system, as many of itscomponents as possible should be traversed by both radiation beams.

Such a dual wavelength scanning device is known from U.S. Pat. No.5,912,868 and is schematically presented in FIG. 1. Paths 1 and 2 of thetwo radiation beams from lasers 3 and 4 are mutually oriented under 90°and combined in a cube beam splitter 5 before entering an objectivesystem 6 that focuses the beams on a record carrier 7. The choice ofwhich radiation source is operated is determined by the type of recordcarrier being scanned. Radiation returning from the record carrier isguided to a photo-detection system 8 via a beam splitter 9. Thephoto-detector transforms the impinging radiation into electricalsignals that represent information stored on the record carrier andtracking information indicating the positional accuracy of the focus ofthe radiation beam on the tracks of the record carrier on which theinformation is written. The tracking relates to tracking in a directionof the optical axis, i.e., focusing, and the tracking in a directionperpendicular to both the optical axis and the direction of a trackbeing scanned in the record carrier. The latter type of tracking is alsoreferred to as radial tracking where it relates to disc-shaped opticalrecord carriers.

The tolerance in the mutual position of the two lasers is relativelytight in view of the accuracy with which the radiation beams must fallon the detection system. Errors in the position on the detection systemmay cause errors in the tracking information. FIG. 2 shows a scanningdevice known from Japanese Patent Application No. JP-A 10326428, inwhich the two lasers are arranged close together, drawn as a singlecomponent 10, making it easier to keep their mutual position within thetolerance. The paths of the two radiation beams are at an acute angle,and the beams are combined by a grating 11. The grating diffracts bothincident radiation beams in transmission. The zero^(th)-order beam ofone of the radiation beams passes from the grating to the objectivesystem, whereas the first-order beam of the other radiation beam passesalong the same path to the objective system.

Japanese Patent Application No. JP-A 10261241 discloses a grating forcombining the radiation beams from the lasers. The grating is optimizedfor transmission of the radiation of 650 nm wavelength in the zero^(th)order and radiation of 780 nm in the first order. The ruling of thegrating is in the form of a series of adjacent saw-tooth profiles, eachsaw-tooth being approximated by a stepped profile.

SUMMARY OF THE INVENTION

Since a high transmittance of the grating is desirable for reading ofand, in particular, writing information in the record carrier at a highdata rate, it is an object of the invention to provide a dual wavelengthscanning device having a grating as beam combiner which has a highertransmittance. It is also an object to reduce the complexity of theruling of the grating to facilitate its manufacture.

This object is achieved if, according to the invention, the scanningdevice is provided with a grating having alternating slanting groovesand slanting lands. The transmittance can be made more than 80% for boththe zero^(th) order beam of the first wavelength beam and the firstorder beam of the second wavelength beam. The reduced complexity of theruling simplifies the manufacture of the grating. Since the ruling hasfewer edges than the known grating, it can be made more accurately. Theimproved accuracy reduces the amount of stray light caused by thegrating, and, hence, increases the transmittance of the grating.

The transmitted power of the grating in the zero^(th) order beam isincreased, when the grooves have an optical depth substantially equal toan integer number times the first wavelength and, at the same time, anodd integer number times half of the second wavelength. The term“substantially equal” means equal to within +/−0.2 wavelengths.

If the grating is a transmission grating provided in a plane, thegrooves and lands slant preferably with respect to the plane at an angleapproximately equal to the angle between an undiffracted beam and aselected diffracted beam from the second radiation beam. The term“approximately equal” means equal to within +/−50%, preferably within15%. The slant of the grooves and lands increases the transmittance ofthe first-order beam diffracted from the second-wavelength beam.

If the grating is a reflection grating provided in a plane, the groovesand lands slant preferably with respect to the plane at an angleapproximately equal to ¼ times the angle between an undiffracted beamand a selected diffracted beam from the second radiation beam.

The alignment of the radiation beams coming from the record carrier andincident on the detection system may be facilitated by arranging theradiation source and the grating mutually adjustable. In addition, thegrating and the photo-detection system can be made mutually adjustable.The two degrees of freedom required for proper alignment are a rotationof the grating around the optical axis and a change of the position ofthe grating along the optical axis.

When the grating is arranged immediately behind the radiation source,both beams follow the same path from the grating to the photo-detectionsystem. Alternatively, when the grating is arranged in front of thephoto-detection system, the beams form spots at the same location on thephoto-sensitive area of the photo-detection system; between theradiation source and the grating the beams follow different opticalpaths, albeit substantially parallel.

In another aspect of the invention, an optical grating for unifyingpaths of a first radiation beam of a first wavelength incident along afirst path and a second radiation beam of a different, second wavelengthincident along a different, second path such that an undiffracted beamfrom the first radiation beam and a diffracted beam from the secondradiation beam substantially coincide, is characterized in that thegrating has alternating slanting grooves and slanting lands. Preferably,the grooves have an optical depth substantially equal to an integernumber times the first wavelength. More preferably, the grooves have, atthe same time, an optical depth substantially equal to an odd integernumber times half the second wavelength. This condition optimizes thediffraction efficiency for both wavelengths in the desired direction.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of the invention will become apparent from thefollowing description, given by way of example only, of preferredembodiments of the invention, which refers to the accompanying drawings,wherein:

FIGS. 1 and 2 show schematically prior art scanning devices;

FIG. 3 shows a scanning device according to the invention;

FIG. 4 shows the profile of a beam combining grating according to theinvention;

FIG. 5 shows a further embodiment of the scanning device; and

FIG. 6 shows a scanning device as in FIG. 3 with a reflective grating.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3 shows a scanning device according to the invention for scanningan optical record carrier 20. The record carrier comprises a transparentlayer 21, on one side of which an information layer 22 is arranged. Theside of the information layer facing away from the transparent layer isprotected from environmental influences by a protection layer 23. Theside of the transparent layer facing the device is called the entranceface 24. The transparent layer 21 acts as a substrate for the recordcarrier by providing mechanical support for the information layer.Alternatively, the transparent layer may have the sole function ofprotecting the information layer, while the mechanical support isprovided by a layer on the other side of the information layer, forinstance, by the protection layer 23 or by a further information layerand a transparent layer connected to the information layer 22.Information may be stored in the information layer 22 of the recordcarrier in the form of optically detectable marks arranged insubstantially parallel, concentric or spiral tracks, not indicated inthe Figure. The marks may be in any optically readable form, e.g., inthe form of pits, or areas with a reflection coefficient or a directionof magnetization different from their surroundings, or a combination ofthese forms.

The scanning device comprises a radiation source 25 that can emit afirst radiation beam 26 having a first wavelength and a second radiationbeam 27 having a second wavelength. The dashed lines 28 and 29 indicatethe principal rays of the radiation beams 26 and 27, respectively.Although these radiation beams travel through the same optical elements30, 31, 34, 35, 43 from the radiation source to the detection system,they are said to follow different paths because their principal rays donot coincide. The radiation beam 26 follows a first path and theradiation beam 27 follows a second path through the optical elements.

The radiation source may include a first and a second semiconductorlaser emitting at nominal wavelengths of 650 and 780 nm, respectively.The first laser will be operated when the record carrier being scannedis of the DVD type and the second laser will be operated when the recordcarrier is of the CD type. The two lasers may be integrated on onesemiconductor chip, allowing a distance of the emission points of thelasers of the order of 100 μm.

A beam splitter 30 reflects the diverging radiation beams 26 and 27towards a collimator lens 31, which converts the diverging beams 26 and27 into collimated beams 32 and 33. After reflection on a mirror 34, thecollimated beams are incident on an objective system 35. The objectivesystem may include one or more lenses and/or a grating. The objectivesystem 35 changes the beams 32 and 33 to converging beams, incident onthe entrance face 24 of the record carrier 20. The objective system hasa spherical aberration correction adapted for passage of the radiationbeam through the thickness of the transparent layer 21. The convergingbeams form spots 36 and 37 on the information layer 22. Radiationreflected by the information layer 22 forms diverging beams, transformedinto substantially collimated beams by the objective system 35 andsubsequently into converging beams 38 and 39 by the collimator lens 31.The beam splitter 30 separates the forward and reflected beams bytransmitting at least part of the converging beams from the recordcarrier towards a detection system 40. The detection system captures theradiation and converts it into electrical output signals 41.

A signal processor 42 converts these output signals to various othersignals. One of the signals is an information signal, the value of whichrepresents information read from the information layer 22. Theinformation signal is processed by an information processing unit forerror correction. The signal processor 42 also provides other signals,such as the focus error signal and a radial error signal. The focuserror signal represents the axial difference in height between the spot36 or 37 and the information layer 22. The radial error signalrepresents the distance in the plane of the information layer 22 betweenthe spot 36 or 37 and the center of a track in the information layer tobe followed by the spot. The focus error signal and the radial errorsignal are fed into a servo circuit for controlling a focus actuator anda radial actuator, respectively. The actuators are not shown in theFigure. The focus actuator controls the position of the objective system35 in the focus direction, thereby controlling the actual position ofthe spot 36 or 37 such that it coincides substantially with the plane ofthe information layer 22. The radial actuator controls the position ofthe objective lens 35 in a radial direction, thereby controlling theradial position of the spot 36 or 37 such that it coincidessubstantially with the central line of track to be followed in theinformation layer 22. The tracks in the Figure run in a directionperpendicular to the plane of the Figure.

A beam combiner 43 in the form of a grating, arranged in the opticalpaths of the beams before the detection system 40, forms an undiffractedbeam 44 from the beam 38 and a first-order diffracted beam 45 from thebeam 39. The beams 44 and 45 converge at substantially the same locationon the photo-sensitive surface of the detection system 40. Hence, only asingle detection system and accompanying electronics is required for thetwo different radiation beams.

Although the scanning device shown in FIG. 3 uses the first-order beam45 diffracted from the beam 39 and the undiffracted beam 44 from thebeam 38, other combinations of diffracted orders are possible, e.g., afirst-order beam from both beams 38 and 39. The use of an undiffractedbeam has advantages in devices that can write information on the recordcarrier. When a semi-conductor laser is used as radiation source, thewavelength of the radiation shows a small shift, of the order of a fewnanometers, when changing the radiation power from the read level to thehigher write level. If such a radiation beam is diffracted in the firstorder by a grating such as the beam combiner 43, a change in thediffraction angle will occur, resulting in a change in the position ofthe spot on the record carrier and/or the detection system. Therefore,the beam that is switched in power between the read and write level ispreferably transmitted or reflected undiffracted by the beam combinergrating. Hence, in a scanning device that is able to read and write inthe CD format and read only in the DVD format, the 785 nm beam for CDshould be undiffracted and the 650 nm beam for DVD should be diffracted,preferably in the first order.

FIG. 4 shows a cross-section through the profile of an embodiment of thegrating 43 according to the invention in a direction perpendicular tothe grating lines. A grating line has a crenellated profile of a groove50 and a land 51. The flat bottom of the groove and the flat land slantat an angle α with respect to the plane of the beam combiner 43 in whichthe grating is arranged. The angle α is chosen to make the intensitiesof the diffracted beam and the non-diffracted beam equal. For atransmission grating, the angle α is chosen substantially equal to theangle θ, the angle between the diffracted beam and the undiffractedbeam. The sine of the angle θ for a first-order beam diffracted intransmission is equal to λ/(2(n−1)p), where λ is the wavelength of thebeam, n the refractive index of the material of the grating and p thepitch of the grating. This reduces to sin θ≈λ/p for most glasses andplastics, which have n≈1.5. The grating lines have a pitch “p” and adepth “d” as indicated in the Figure. The transmission of the gratingfor the undiffracted beam 43 is optimized by choosing the depth “d” suchthat it corresponds to a phase depth equal to a multiple of 2π for thewavelength of the radiation beam 38. The phase depth for theair-incident transmission grating is substantially equal to 2πd(n−1)/λ,where n is the refractive index of the grating material and λ is thewavelength of the incident radiation.

Instead of a transmission grating, a reflection grating may be used inthe device. This is shown in FIG. 6 in which a scanning devicesubstantially the same as that shown in FIG. 3 uses a reflection grating43′. An air-incident reflection grating has a phase depth substantiallyequal to 4πd/λ. In that case, the angle α should be chosen substantiallyequal to θ/4, which is equal to l/(4p) for a first-order diffractedbeam.

In a particular embodiment of the scanning device, the parameters havethe following values: the first wavelength, i.e., that of the radiationbeam 38, is 650 nm, the second wavelength, i.e., that of the radiationbeam 39, is 785 nm, the refractive index of the grating material is 1.5,the diffraction angle θ for the second wavelength is 1.7° and aseparation between the spots of the radiation beams 44 and 45 on thedetection system 40 without grating 43 equal to 120 μm. The pitch “p”,equal to λ2/sinθ, is now 26.5 μm. The phase depth of the grating for thefirst wavelength is taken to be equal to 6π, corresponding to a depth“d” equal to 3.9 μm. The phase depth of the grating for the secondwavelength is 4.95π. Since this corresponds to approximately anti-phase,the radiation beam of the second wavelength will form a low-intensityundiffracted beam and a high-intensity diffracted beam.

FIG. 5 shows a further embodiment of the scanning device according tothe invention, in which the beams are combined close to the radiationsource instead of close to the detection system as shown in FIG. 3. Aradiation source 60 can emit two radiation beams 61 and 62 of differentwavelength. Both beams are incident on a beam combiner 63 according tothe invention. The beam combiner includes a grating having a profile asshown in FIG. 4. A beam 64 coming from the beam combiner is theundiffracted beam from the incident beam 61 and/or the first-orderdiffracted beam from the incident beam 62. The beam 64 is focused on theinformation layer 22 of the record carrier 20 via a beam splitter 65 andan objective system 66. Radiation reflected by the record carrier istransmitted by the beam splitter 65 and intercepted by a detectionsystem 67. The optical system of the scanning device shown in FIG. 5 isadvantageous compared to that shown in FIG. 3, because the former has asingle optical path through the objective lens for the radiation beamsof different wavelength, thereby reducing the use of the field of theobjective lens.

The optical grating according to the invention may advantageously beused in an optical scanning device as described in the European PatentApplication No. EP01/10737, corresponding to U.S. Patent ApplicationPublication No. 2002/0051247. This device uses a beam combiner havingtwo gratings, one on each side of a plate. One or both gratings may haveslanting grooves and slanting lands.

1. An optical device for scanning an optical record carrier, said optical device comprising: a radiation source for generating a first radiation beam of a first wavelength traveling along a first path and a second radiation beam of a different, second wavelength traveling along a different, second path; a photo-detection system; an optical system for guiding the radiation beams via the optical record carrier to the photo-detection system; and a grating for combining the first and second beams such that they substantially coincide on the photo-detection system, characterized in that the grating has alternating slanting grooves and slanting lands, thereby enabling said grating to have a high transmittance for a zero^(th) order beam of the first radiation beam of the first wavelength and for a first order beam of the second radiation beam of the second wavelength.
 2. The optical device as claimed in claim 1, wherein the grooves have an optical depth equal to substantially an integer number times the first wavelength and, at the same time, substantially an odd integer number times half of the second wavelength.
 3. The optical device as claimed in claim 1, wherein the grating is a transmission grating provided in a plane and the grooves and lands slant with respect to the plane at an angle approximately equal to the angle between an undiffracted beam from the first radiation beam and a selected diffracted beam from the second radiation beam.
 4. The optical device as claimed in claim 1, wherein the grating and the radiation source are mutually adjustably positioned.
 5. The optical device as claimed in claim 4, wherein the grating and the photo-detection system are mutually adjustably positioned.
 6. The optical device as claimed in claim 1, wherein the grating and the photo-detection system are mutually adjustably positioned.
 7. An optical device for scanning an optical record carrier, said optical device comprising: a radiation source for generating a first radiation beam of a first wavelength traveling along a first path and a second radiation beam of a different, second wavelength traveling along a different, second path; a photo-detection system; an optical system for guiding the radiation beams via the optical record carrier to the photo-detection system; and a grating for combining the first and second beams such that they substantially coincide on the photo-detection system, characterized in that the grating is a reflection grating provided in a plane has alternating slanting grooves and slanting lands, thereby enabling said grating to have a high reflectance for a zero^(th) order beam of the first radiation beam of the first wavelength and for a first order beam of the second radiation beam of the second wavelength, wherein the grooves and lands slant with respect to the plane at an angle approximately equal to ¼ times the angle between an undiffracted beam from the first radiation beam and a selected diffracted beam from the second radiation beam. 